1. Field of the Invention
This invention relates to subscriber loop carrier systems and, more particularly, to selective party ringing in digital subscriber loop transmission systems.
2. Description of the Prior Art
In order to provide fully selective ringing signals on multiparty telephone lines, it has been customary to superimpose a direct current voltage on the ringing signal and to vary the polarity and the conductor-to-ground connection to distinguish among the various parties. Thus positive or negative superimposed battery selectively connected between the tip or ring conductors and ground provides four party fully selective ringing. The ringing circuit in each telephone set must, of course, be equipped to respond only to a ringing signal with the proper polarity of superimposed battery and applied between the appropriate conductor and ground. One such ringing signal detector is disclosed in the copending application of C. E. Jones and W. F. MacPherson, Ser. No. 648,165, filed Jan. 12, 1976 and assigned to applicant's assignee.
It has become increasingly desirable to connect local telephone loops to a central office location over carrier systems of one type or another in order either to maximize the use of conductor pairs in congested areas, to avoid laying new cable, or to extend the range of conductor pairs in sparsely populated rural areas. Modern digital technology has permitted the use of digital multiplexing techniques in such carrier systems. Such digital transmission systems also have the advantages of good immunity from noise and the capability of implementation with standard digital circuitry. One such digital subscriber loop carrier system is disclosed in the J. L. Caldwell U.S. Pat. No. 3,924,080, granted Dec. 2, 1975, and assigned to applicant's assignee.
In order to serve multiparty lines over digital transmission systems, it is necessary to transmit the selective ringing signals to the remote end of the transmission system in such a manner that they can be detected and utilized to ring the appropriate party. Moreover, these ringing signals must replicate the standard ringing and silent interval cycle at the subscriber location. Finally, such multiparty ringing must be accomplished with small and inexpensive circuitry to insure the economic viability of the digital transmission system.